ET 51 - Electrician Theory Examination Marking Schedule

Transcription

1 ET 51 - Electrician Theory Examination Marking Schedule Notes:1. means that the preceding statement/answer earns 1 mark. 2. This schedule sets out the accepted answers to the examination questions. A marker can exercise their discretion and decide on the overall accuracy of any answer that is presented in the candidate s own words. 3. Symbols and terms - alternatives Power W or P Voltage V or E or U Phase Active Question 1 (a) To determine whether the fault rating of the protection devices is adequate to safely interrupt a circuit. To find the prospective short circuit current level of the installation. (2 marks) (b) (i) Insulation resistance test (i) and (ii) are interchangeable 1 MΩ or 10,000Ω (ii) Earth leakage test 5 ma (c) No. It does not operate on residual pulsating (1½ marks) direct current. (d) (i) No effect. (ii) The power dissipated drops 10% The power dissipated drops 195W (e) 235 x 12 = 2820VA = 2115 W 2820 VA =

2 Question 1 (f) The main fuse may not operate due to the high impedance of the earth fault loop. Any metal not normally live could be live at up to 230V. The main switch does not isolate the installation. Internal components of equipment are live with control switch in the off position. Fire hazard at a high-resistance joint (g) (i) To ensure the device can safely clear the prospective short-circuit current. To ensure the protective device will operate in the required time (2 marks) (ii) (h) (i) An HRC fuse An ACB The maximum value of torque the motor can produce. (ii) 1500 rpm (i) Any TWO of: A touch voltage hazard between conductive parts and the mass of earth. Protective devices may not operate under fault conditions. Fire hazard at any high resistance joint in the main neutral. Over or under voltage could damage equipment particularly on three phase installations. (2 marks) (j) A thermistor detects a rise in temperature in the motor windings. The change in resistance operates the thermistor monitoring relay which opens the motor control circuit. 2

3 Question 2 (a) (i) An excessive current will flow 0n all phases * The HRC fuses will operate (blow). * The thermal overload will open the control circuit (ii) A sustained high current will flow on all phases * The HRC fuses will operate (blow). * The thermal overload will open the control circuit (iii) A sustained high current will flow on two phases * The HRC fuses will operate (blow). * The thermal overload will open the control circuit (b) (i) The thermal overload device would operate Within 2.5 to 3.5 minutes. (ii) The HRC fuses would operate Within about 0.2s (iii) The 50A fuse with 75A flowing will operate in about 10 minutes. The 50A fuse with 150A flowing will operate in about in 0.2s. 3

4 Question 3 (a) Star point earthed No marks can be For each switchboard - an earthing conductor connected to the earth bar. awarded for part (a), if there are any of the following hazards: For each switchboard - an MEN link between the neutral bar and earth bar. For each switchboard a neutral conductor between the neutral bar and the distribution neutral. A phase connected to a neutral bar No MEN link in either switchboard. No main earth on either switchboard The star point of the transformer is not earthed (b) The protective earthing conductor of the socket outlet final subcircuit The earth bar. MEN link and neutral bar The main neutral conductor The distribution neutral conductor to the star point of the transformer The transformer winding The distribution phase conductor The main phase conductor The main switch The final subcircuit protective device The phase conductor of the socket outlet final subcircuit (c) It is part that has the lowest impedance. 4

5 Question 4 (a) No marks can be awarded for part (a), if E N L L L there are any of the following hazards: Tripping Device Test button A short circuit No test resistance. The socket outlet is not earthed The test button circuit is connected to the supply side of the tripping device Test resistance Sensing coil/toroid Three-phase socket Correctly connected phases, neutral (1½ marks) and earth. Correctly connected test button and resistance Correctly connected sensing coil/toroid Correctly connected tripping circuit Correctly connected socket Working circuit (1½ marks) (b) It is the current imbalance required to trip the RCD It is the rated residual current of the device (c) * There is an imbalance between the neutral current and phase 5

6 Question 4 current * There is earth leakage current in the circuit Greater than the residual tripping current of the RCCB (d) RCBO Residual Circuit Breaker with overload (overcurrent) protection SRCD Socket Residual Current Device 6

7 Question 5 (a) (i) It shows that when the fault current rises The time taken for a fuse to blow shortens (ii) 25A fuse 50A fault current - would operate in about 60 seconds. 25A fuse - 200A fault current - would operate in about 0.08 seconds. (iii) 20 x 1.5 = 30A (iv) 1000 seconds (v) 90A (b) This is the maximum fault current the fuse can safely interrupt. (c) This is the time it takes to interrupt the flow of current and extinguish the arc. It is the sum of the pre-arc time and the arc time (d) This is the maximum current that the fuse is designed to carry continuously 7

8 Question 6 (a) V s = N s x V p Use of 230V in N p calculations is acceptable = 1 x = V (b) V 1 = V ph x 3 Use of 400V in calculations is acceptable = x 3 = 398.6V (c) kva = V 1 x I L x = x x = kva (d) I L = VA (V 1 x 3) = (398.6 x 3) = A (e) Regulation % = VNL VFL x 100 VNL = x = 3.48% 8

9 Question 7 (a) Load 2 VAr = kva 2 - P 2 = = VAr Load 3 VAr = kva 2 - P 2 = = VAr (b) Horizontal Vertical Watts (P) Var (Q) Load Load Load Totals Total VA kva = P 2 + VAr 2 = = = kva (c) Pf = W VA = =

10 Question 8 (a) (i) The motor is connected in delta with a line voltage of 400V (ii) L1 L2 L3 U V W W1 U1 V1 Correctly connected windings Links connected to achieve a delta configuration Links connected to achieve a three-phase supply (b) (i) To prevent the hoist starting if two phases on the supply were reversed. (ii) The hoist will go up if the down control is operated and will go down when the up control is operated. The upper and lower limits on the hoist become ineffective. (2 marks) (c) The thermal overload protects against a sustained current rise. In excess of overload current settings. The thermistor protects against excessive temperature rise in the motor. Regardless of whether the current rises. (d) (i) As the temperature (of a motor) increases the resistance of the thermistor falls. (ii) As the temperature (of a motor) increases the resistance of the thermistor rises. 10

11 Question 9 (a) kva = V L x I L x = 400 x 142 x = kva (red phase) (b) The transformer is 100kVA (c) I PH = kva V L x 3 = 100, x 3 = A (d) I L = kva V L primary x 3 = 100, ,000 x 3 = 5.24A (e) (i) A tap-changer (ii) On the high-voltage side The current switched by the tap-changer is small. (1½ marks) 11